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NBME 23 Answers

nbme23/Block 1/Question#33

A 25-year-old woman and her 25-year-old husband ...

α-Thalassemia trait

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 +7  upvote downvote
submitted by nwinkelmann(153),

I found a picture showing the transmission by two heterozygous alpha thalassemia trait asian (cis-deletion) parents, and modified it to also show the inheritance by two heterozygous alpha thalassemia african (trans-deletion) parents. Here you go: https://drive.google.com/open?id=1HQU1VuhI4Jc9WJR2POwnTq9T8wEdv7dw

 +4  upvote downvote
submitted by asapdoc(26),

Dr.Goljan explains it really well on the audio. I will just give the basic idea. All the hemoglobins have alpha in it therefore you cannot notice it on hemoglobin electrophoresis

someduck3  Just to add to this; a-thal is due to a deletion. While b-thal is due to a mutation. If they had a b-thal there would be target cells. a-thal just presents as microcytic & hypochromic. +2  
almondbreeze  looks like a-thal can have target cells too. Individuals with alpha thalassemia trait (-α/-α or --/αα) are asymptomatic, with a normal CBC. The peripheral blood smear typically shows hypochromia, microcytosis, and target cells. (emedicine.medscape.com › article › 955496-clinical) +  

bc the parents are asian, they are cis deletion carriers. So their kid can either be a cis carrier or have Hb Barts, its not possible to get HbH

 +2  upvote downvote
submitted by krewfoo99(21),

In what situations will HbH be formed (3 alpha chain deletions)?

ergogenic22  one parent has 2 deletions on the same gene, the other parent has 1 deletion, and the offspring receives all three. In this question, both parents have alpha 1 deletion +  
ergogenic22  actually its possible that they both have 2 gene deletions, but regardless, a-thalassemia trait is more likely +  
ergogenic22  and someone above said Asian people are cis-2 deletion so the offspring will not receive two deletion from one parent +  
ergogenic22  ↑↑ I made a mistake by confusing trans and cis cis has deletions on the same chromosome and can pass two deletions to off spring, therefore a chance of allowing HbH +  

 +0  upvote downvote
submitted by nor16(22),

in ß Thalassemia there is HbA2 increase and HbA decreases, even in ß+ , normal electrophoresis rules this out. same for A)-C)